Traditional cancer chemotherapy relies on the ability of drugs to kill tumor cells in cancer patients. Unfortunately, these same drugs frequently kill normal cells as well as the tumor cells. The extent to which a cancer drug kills tumor cells rather than normal cells is an indication of the compound's degree of selectivity for tumor cells. One method of increasing the tumor cell selectivity of cancer drugs is to deliver drugs preferentially to the tumor cells while avoiding normal cell populations. Another term for the selective delivery of chemotherapeutic agents to specific cell populations is "targeting". Drug targeting to tumor cells can be accomplished in several ways. One method relies on the presence of specific receptor molecules found on the surface of tumor cells. Other molecules, referred to as "targeting agents", can recognize and bind to these cell surface receptors. These "targeting agents" include, e.g., antibodies, growth factors, or hormones. "Targeting agents" which recognize and bind to specific cell surface receptors are said to target the cells which possess those receptors. For example, many tumor cells possess a protein on their surfaces called the epidermal growth factor receptor. Several growth factors including epidermal growth factor (EGF) and transforming growth factor-alpha (TGF-alpha) recognize and bind to the EGF receptor on tumor cells. EGF and TGF-alpha are therefore "targeting agents" for these tumor cells.
"Targeting agents" by themselves do not kill tumor cells. Other molecules including cellular poisons or toxins can be linked to "targeting agents" to create hybrid molecules that possess both tumor cell targeting and cellular toxin domains. These hybrid molecules function as tumor cell selective poisons by virtue of their abilities to target tumor cells and then kill those cells via their toxin component. Some of the most potent cellular poisons used in constructing these hybrid molecules are bacterial toxins that inhibit protein synthesis in mammalian cells. Pseudomonas exotoxin A (PE-A) is one of these bacterial toxins, and has been used to construct hybrid "targeting - toxin" molecules (U.S. Pat. No. 4,545,985).
PE-A is a 66 kD bacterial protein which is extremely toxic to mammalian cells. The PE-A molecule contains three functional domains: 1.) The amino-terminal binding domain, responsible for binding to a susceptible cell; 2.) The internally located "translocating" domain, responsible for delivery of the toxin to the cytosol; 3.) The carboxy-terminal enzymatic domain, responsible for cellular intoxication. PE-A has been used in the construction of "targeting-toxin" molecules, anti-cancer agents in which the 66 kD molecule is combined with the tumor-specific "targeting agent" (monoclonal antibody or growth factor). The "targeting-toxin" molecules produced in this manner have enhanced toxicity for cells possessing receptors for the "targeting agent".
A problem with this approach is that the PE-A antibody or growth factor hybrid still has a reasonably high toxicity for normal cells. This toxicity is largely due to the binding of the hybrid protein to cells through the binding domain of the PE-A. In order to overcome this problem, a protein was recombinantly produced which contains only the enzymatic and "translocating" domains of Pseudomonas exotoxin A (Hwang et al., Cell, 48:129-137 1987). This protein was named PE.sub.40 since it has a molecular weight of 40 kD. PE.sub.40 lacks the binding domain of PE-A, and is unable to bind to mammalian cells. Thus, PE.sub.40 is considerably less toxic than the intact 66 kD protein. As a result, hybrid "targeting-toxin" molecules produced with PE.sub.40 were much more specific in their cellular toxicity (Chaudhary et al., Proc. Nat. Acad. Sci. USA, 84: 4583-4542 1987).
While working with PE.sub.40, it was found that the cysteine residues at positions 265, 287, 372 and 379 (numbering from the native 66 kD PE-A molecules; Gray et al., Proc. Natl. Acad. Sci., USA, 81, 2645-2649 (1984)) interfered with the construction of "targeting-toxin" molecules using chemical conjugation methods. The reactive nature of the disulfide bonds that these residues form leads to ambiguity with regard to the chemical integrity of the product "targeting toxin".